Electrolytic polishing of metals



June 1956 E. E. DARMOIS ETAL 2,752,304

ELECTROLYTIC POLISHING OF METALS Filed Nov. 9, 1951 3 Sheets-Sheet 2 60 q in 14,: J5 4&5 4'9 90 r .0: 100

F494 fimtw Emile Egan: Dh'mois Izwal g/belcin J Daub- 5e 0 Wang? K June 26, 1956 E. E. DARMOIS ETAL ELECTROLYTIC POLISHING OF METALS 3 Sheets-Sheet 5 Filed Nov. 9 1951 ELECTROLYTIC POLISHING F METALS Emile Eugene Darmois and Izrail Epelboin, Paris, France,

assignors to Centre National de la Recherche Scientifique, Paris, France Application November 9, 1951, Serial No. 255,606 Claims priority, application France November 10, 1950 2 Claims. (Cl. 204--140.5)

The present invention relates to improvements in the electrolytic polishing of metals.

Electrolytic polishing is selective electrolytic solution which permits of polishing, cleaning or brightening the article connected to the anode. Hitherto, acids have especially been used in the constitution of the polishing baths. These baths are of two kinds. Industrial baths, which generally work at a low potential less than 15 volts, have for the most part a basis of phosphoric acid, chromic acid, sulphuric acid and so forth. They necessitate, after polishing, very careful rinsing and drying of the article, which in most cases, however, does not sufiice to avoid the rendering passive of the surfaces, this being often very deleterious for example in applications to the electroplating and electronic fields. T avoid this passivation, recourse has been had to baths with a perchloric acid basis, but these baths necessitate potentials of 20 to 100 volts and have the inconvenient feature of being very readily explosive. Serious accidents have already been produced with mixtures of perchloric acid and acetic anhydride, and in spite of marked advantages, such as improvement of reflecting power or of magnetic permeability, they are used slightly and only in laboratories where they should even then be handled with adequate precautions (the content of perchloric acid ought not to exceed 40%, the temperature ought not to exceed 40 C. and it is necessary to avoid any contact with organic substances).

As a result of research, the inventors have discovered that electrolytic polishing is associated with the creation, in the region of the anode, of intense electric fields produced by anions of small dimensions adsorbed upon the anode. The field of these anions draws out metallic ions and this field is so much the more intense the less the electrolytic bath contains water in the free state (that is to say of water not linked to the ions).

According to this invention use is made essentially for the constitution of polishing baths, not of the acids employed hitherto, but of salts whose anions present the characteristic property of possessing, in electrolytic solution in the bath in which they are dissolved, an effective radius less than that resulting from measurements effected upon the salt in the crystalline state.

In order to determine which anions possess this characteristic property, the following procedure may be adopted: The effective radius I's of the anions in solution is calculated by applying Stokes law. Thus, in Angstroms:

1 represents such a graph for the anions of salts in solu- Patented June 26, 1956 tion in water and Fig. 2 for the same anions in solution in methyl alcohol.

The typical points situated below the bisector OT correspond to the ions for which rs rc. These anions can therefore be utilised, in accordance with the invention, for the constitution of electrolytic polishing baths. On the other hand, the anions whose typical points are situated above this bisector are not suitable for the constitution of these baths. It will be seen from Figs. 1 and 2 that certain anions which are suitable in aqueous solution may not be suitable in alcoholic solution and conversely. For example, the chlorides (Cl ion) and the bromides (Br ion) are suitable in aqueous solution and not in alcoholic solution. However, the perchlorates (C10 ion) are suitable both in aqueous solution and in alcoholic solution.

According to the Formula 1, it will be seen that Ts is the smaller, the larger 1 is. It is therefore desirable to utilise a solvent of as high viscosity as possible. As it is also of advantage to include little water in the free state, a solvent may be used which is hygroscopic (for example, acetic anhydride, acetic acid or alcohols).

It will be realised that it is characteristic of the invention to use, as a polishing bath, a salt Whose anions satisfy the condition F. Or a mixture of salts having similar anionic property may be used.

Another characteristic of the invention is, therefore, the selection, preferably, of the cation or cations in such a manner that the eitective radius rs thereof in the salt solution in the bath under consideration is larger than the radius re thereof in the crystalline state. The cation or cations should therefore be selected so that rs rc. The larger the ratio is, the better will be the results, naturally without taking account of the action which this cation or cations may exert upon the salt, of the metal to be polished, in solution in the bath in the course of the operation (formation of complex compounds, precipitates and so forth).

By way of example, it will be seen from Fig. 1 that, in aqueous solution, the salts of potassium are, all other things being equal, to be rejected; the salts of sodium or silver are satisfactory; the salts of copper, iron and magnesium are well suitable; the salts of aluminum are highly highly to be recommended.

By way of non-limitative example, some compositions of baths in conformity with the above characteristics have been indicated in the ternary diagram shown in Fig. 3. These baths possess the property of not rendering the surfaces passive, of not including free perchloric acid and of not being explosive.

The baths represented are formed by a solution of a perchlorate in acetic acid. In the diagram, there have been represented:

From C to D the proportionof C104, expressed by the corresponding proportion of ClO-rH (from 0% to 20%) From D to E the proportion of water (from 0% to 20%) From E to C the proportion of acetic acid (from to For a given metal and for a given electrolyte of composition in accordance with the invention, there exist many compositions of baths which give excellent results and which are designated by the term preferable compositions. The inventors'have observed that if all thepreferable compositions of baths are considered in relation to a large number-of metals, for various electrolytes including a salt or salts whose anions present the characteristic property above referred to, then all the preferable compositions group themselves on straight lines which converge towards the point Got the diagram and it will be seen that each of these straight lines includes solely the preferable compositions with respe-ct'to the luse of the same one electrolyte.

Thus, with perchlorate of sodium, the points corresponding to thepreferable compositions are'grouped, in the example.represented,"on the' straight lines I and III.

With perchlorate 'ofma'gnesiumusedas the salt included in the composition"ofthebath, the typical'pointsof the preferable compositions are found'on the straightline IV.

By way 'of comparison, ibaths=giving good "results'with pureperchloric acid used instead of a perchlorate have been indicated'in the diagram. These points are' likewise groupcdon a straightlinetliue II).

However, it should be understood that the invention does not contemplate bathsusing uncombined perchloric cid.

if the various straight lines passing through the point C and corresponding to the preferable compositions of baths including a salt as electrolytein accordance with the invention are thus traced, there. exists on each of these lines a point corresponding'to a preferable composition 'of the baths for a given metal.

The inventors have noted that, if the. points in relation to the same one metal for a given electrolytic salt (for instance perchlorate of sodium or perchlorate ofmagnesium) are known on two of these straight lines, it is sutlicient to join thesetvvo points to obtaina preferable composition 'of bath' for the same metal'on all of the straight lines corresponding to the same salt.

This is how the diagram of Fig. 3 is used:

For example, if one desires to know a preferable composition of new bath for the polishing of cobalt, constituted by a solution of'perchlorate of sodium in acetic acid,

it is seen (point Co, line'I) that a desirableproportion is a quantity of perchlorate corresponding to-11.6% of perchlcric acid, 84.5% of -acetic acid and 4% of Water.

It-is also possible (line Ill) to take a quantity of perchlorate equivalent to 7% of perchloric acid, 89% of acetic acid and 4% of Water.

A preferable composition of new polishing bath for nickel, composed of perchlorate of magnesitun in solution in acetic acid is (point Ni on'li-ne' IV):

Perchlorate, quantity corresponding to Acetic acid 88.5% Water 6.0%

C-e represents the: proportion of *perchlorieacidccorresponding' to the quantity of :perchlorateqcontaine'dwin the bath.

.Ed indicatestheproportion of aceticacid;

D--c represents the proportionsof'water.

5.5% of acid the apices of the quadrilateral have the following onordinates:

.Equivalent Acetic tn 0104K acid 33 (C E) (ED) Accordingly,'the inventioncontemplates more particularly any electrolytic polishing bath composed of aperchlorate, water and-acetic acid whose compositionis represented by .a point situated in the quadrilateral thus defined. Inparticular, the proportion of acetic acid is comprised between and 82%.

Second rule-Having determined experimentally a preferable composition of a bath, whose composition includes a given solvent and a salt of an anion such that and, in. particular, a perchlorate, for the electrolytic polishing of a given metal, the typical points of all the preferable compositions of baths for the polishing of other metals, using thesameanion and the same solvent, are .found onithe straight .line which joins to the point C the typical point .of the first bath determined.

.Inypractice, a preferable composition of a bath of the nature.indicated,-:isdetermined by'trial and error. The typical point .of this bath-is plotted upon adiagram of the-kind shownin Fig. 3. -:This point is joined :to the apex C of the triangle. ;In order to determine the compositions -.of;;prefer-able baths relating to a given solvent, anion and metal or alloy, it is then sufiicient to-study baths .Whose typical pointsare .found regularly spaced upon the straight line thus drawn.

- Third rule-For the-various straight lines such as I,

.HLIV and so forth, the relationof the distances upon the various lines of the two points corresponding to two given metals is constant, it the conditions in which the bath exists .during the operation are the same. For example, on the lines I and IV, the relation the following: Determining a preferable composition of bath including the given salt and solvent, for two metals other than .the. given metal; drawing the straight line which joins these two'points; determining preferablecompositions of baths includingthe given solvent but another salt ;of the same anion for these two other metals and for the given metal; drawing the straight line which joins these three points; and selecting upon the straight line provided by the first determination a point for which the relation.ofthedistances .toithe point'in the sameJine relating to the two other metalsis the same as the relation of the distances of the point relating to this given -metal and thepoints relating to these two other metals :upon. the-straight line resulting from the second determina- .tion.

Onthe other hand due to-the formation of a preare not the same.

Fourth rule.For the polishing of an alloy, every point corresponding to a preferable composition of bath divides the segment which separates the typical points of the two metals of the alloy in a relation corresponding to the proportion of these metals in the alloy.

Thus, for the line IV, the point marked Fe-Ni corresponds to an alloy of equal parts of iron and nickel. It is equidistant from the points Fe and Ni.

Consequently, if the points corresponding to the two metals forming an alloy are known, it is sufiicient to select the point dividing, in the desired proportion, the segment which joins the points corresponding to these two metals.

In practice, this amounts to leaving constant the I relation anion solvent and modifying only the proportion of water.

The above rules apply Whatever may be the solvent used in the bath.

If one changes from one solvent to another, for instance from acetic acid to ethyl alcohol, the points corresponding to the optimum proportions are always contained in a quadrilateral such as MNRQ, but all other things remaining equal, it will be seen that the points mn and rq of intersection of the straight lines MN and RQ with CE are located on a straight line CE. For ex-. ample, if use is made of ethyl alcohol as solvent, these points are comprised between 80% and 92% of alcohol by weight. The four apices of the quadrilateral MNRQ suffer the same translation parallel to CE. The diagram relating to ethanol is shown in Fig. 3a.

The inventors have noted that the baths with a perchlorate basis polish copper and its alloy badly. In this case, according to the invention, the perchlorates are replaced by the phosphates. The latter salts have the advantage of being capable of being used not only in acetic or alcoholic solution but also in aqueous solution. In the latter case, the best proportions are comprised between 100 gr. and 900 gr. of anion, P04, per litre according to the nature of the metal to be polished or the cation of the phosphate employed. In particular, for the polishing of copper in an aqueous solution of (NI-I4)3PO4, a recommended proportion corresponds to 266 gr. of this salt per litre, that is to say 167 gr. of anion, P04, per litre.

It is expressly stated that, in all which precedes, the perchlorates have been indicated by way of example only.

The same things could be said for all the electrolytic polishing baths comprising, as electrolyte, any salt whose anion satisfies the condition in the solution in question.

All that follows concerns the new baths, that is to say comprising in practice, in known manner, some grammes per litre of the metal to be polished dissolved in the bath.

The inventors have observed that the proportions of the three constituents of the bath; salt, solvent and water, which are the best for the new bath do not remain the best when the bath has dissolved a more or less large proportion of the metal to be polished. It has been observed that, in order to preserve, for the baths according to the invention, their optimum properties as they are used, it is sufficient to add a quantity of solvent substantially proportional to the quantity of metal dissolved.

Experience has shown that this quantity of solvent is given by the formula x='K (p-n), where x indicates, as a percentage of the whole of the bath, the quantity of solvent to be added,

K is a constant which depends upon the bath and the metal polished, n is another constant which depends on the bath and the metal polished (this is the minimum quantity of the metal polished to be dissolved in the bath to start off the polishing), v

p is the weight of metal dissolved per litre of bath.

Diagrams giving x as a function of p for a bath composed of sodium perchlorate dissolved in acetic acid are represented in Fig. 4.

The curve (I) concerns the polishing of nickel.

The curve (II) concerns the polishing of iron.

It will be seen that in this bath n=4.5 approximately for the iron and the nickel; K=% for the iron and A for the nickel.

Consequently, in practice, it will be sufiicient to trace the curve relating to the bath in question and for the metal which it is desired to polish and to know, at any instant, the quantity of metal already dissolved by the bath in the course of the polishing operations. This quantity will result either from weighing of the articles before and after polishing or, more simply, from measurement of the number of ampere hours which have passed through the polishing tank since the new bath was put into service.

The quantity of solvent to be added periodically to the bath for maintaining it in condition will thus be known. It is even possible to envisage that the current passing in the tank should act automatically upon a distributor regulating the supply of the solvent from a reservoir to the tank. Actually, in practice, the quantity of solvent to be added is, at any instant, proportional to the intensity of the current.

An arrangement may also be conceived in which the additions of solvent will be made periodically to the bath every time the quantity of dissolved metal is increased by a certain amount, for example 5 gr. per litre. In this case an ampere-hour meter would release weighing means of a vessel containing a measured quantity of solvent each time the number of ampere hours supplied to the tank was increased by a fixed amount corresponding to 5 gr. per litre.

Consequently it is important, according to the invention, to utilise, as cation of the salt whose anion satisfies rs rc, a cation capable of precipitating the polished metal in order to liberate the anion which was combined with this metal. For polishing iron, use is made of a solution of perchlorate of soda in order that the iron dissolved by the electrolysis may be precipitated by the soda in the form of hydrate of iron insoluble in the bath.

However, it is also necessary to avoid the condition that the typical point of the composition of the bath does not come in the region situated (Fig. 3) in the vicinity of the point C. Such a point (for example U) corresponds to a very dilute bath and it is obvious that it will no longer attack the metal.

What we claim is:

l. Electrolytic bath for polishing metals constituted solely by a solution of a salt of perchoric acid in ethyl alcohol, said solution comprising from to 92% of ethyl alcohol by weight, cation of said salt satisfying the inequality rs lc in which rs is the radius of the cation in the said solution, and To is the radius of the cation as determined by measurements made upon the salt in the crystalline state.

2. Electrolytic polishing bath for the metals belonging to the 4th series of group VIII of Mendeleefs periodic table of elements, which consists solely of a solution of a salt of perchloric acid in ethyl alcohol, said solution comprising from 80% to 92% of ethyl alcohol by weight, the metal of said salt being selected from the group consisting of. sodium, silver, copper, ,iron,,m gnesium, andgaluminum.

References Cited in the file of this patent Mateosian July 25, 1950 10 Emery Jan. 8, 1952 :8 OTHER REFERENCES Chemical-Abstracts, vol. 41 (1947),page 3382, abstract of paper entitled Preparation of small surfaces by electropolishing, by KnuflrWintcrfeldt.

Proceedings of the American Electroplaters Society, 1941, pages 109, 110, 111 ,ofarticleby Faust et a1.

Physical Chemistry of Electrolytic Solutions, by Harned et 211., 2nd edition (1950), pages 183, 184. 

